Process of casting nodular iron



PROCESS OF CASTING NODULAR IRON Harold N. Bogart and Robert B. Melmoth, Detroit, Mich., assignors to Ford Motor Company, Dearborn, Mich, a corporation of Delaware No Drawing. Application September 28, 1950, Serial No. 187,350

3 Claims. (Cl. 75-130) This application is concerned with the founding industry and more particularly with a method for improving the physical properties of cast nodular iron. This socalled nodular iron has received recent widespread publicity both in the current and patent literature and it is thought to be unnecessary to describe it further other than to state that it may be produced by adding sufiicient magnesium to an appropriate gray iron melt to produce a magnesium residual of about 0.05% and inoculating the melt just prior to pouring into the mold with any of the many commercially available graphitizing inoculants. This treatment, if properly carried out, will result in an as cast product in which the uncombined carbon is present in the form of spherulites or nodules rather than the flaky form which is characteristic of typical gray iron. The matrix may be either ferritic or pearlitic, depending upon the analysis and casting conditions. As is now well understood in the art the absence of flaky graphite permits the production of the casting having a much higher tensile strength and ductility than ordinary gray iron.

The production of nodular iron castings particularly in highly irregular shapes has been impeded by the persistent occurrence of a phenomenon referred to in the industry as cope defects. This name has been applied because of the tendency of this defect to occur at the upper portion of the castings. A casting seriously weakened by these cope defects may present to the eye a perfect surface and also appear radiographically sound. However, when fractured the. castings will exhibit a large number of inclusions of unknown composition surrounded by an envelope of soft graphite. These inclusions may vary in size from those barely visible to the naked eye, up to one quarter of an inch in diameter. The presence of this graphite film or envelope, of course, presents a plane of discontinuity and weakens the castings. These inclusions have been found particularly troublesome in the casting of crankshafts for internal combustion engines inasmuch as they tend to congregate at the juncture of the pin bearing and check, which is precisely the location of maximum stress concentration at which fatigue cracks tend to develop.

The inventors have discovered that this phenomenon of cope defects can be substantially eliminated by the application to the molten metal of any of the many fluxes which have been developed for use with magnesium and which have the common property of wetting magnesium oxide at temperatures encountered in gray iron founding. Particularly good results have been obtained with boric acid, borax, fused borax, and alkali metal fluoborates, as well as the fluxes which are based upon the double chloride of an alkali metal and magnesium. For a detailed description of commercially available fluxes, reference is made to the Metals Handbook, 1948 edition, page 974.

These fiuxing materials must be brought into intimate contact with the molten metal, and the molten metal should be protected from contact with air or oxidizing mold gases to the greatest possible extent after the flux treatment. These fluxes may be poured into the mold in the granular form before the metal is poured, or they may be applied to the molten stream as it is poured into the mold. If molding practices permit, the fluxing material may be incorporated into the molds per se, or may be dusted upon the surface of the mold destined to contain the molten metal. On large scale work it has been found preferable to apply the fluxing material to the metal in the ladle, care being taken to secure intimate contact of the flux with the molten metal.

In the application of the process the inventors have been singularly successful in the production of internal combustion engine crankshafts having the following composition:

Percent Carbon 3.50-3.80 Silicon 2.20-3.00 Manganese 0.30-0.50 Phosphorus 0.100 Sulphur 0.017 Magnesium 0.050 Iron Remainder These crankshafts weigh about sixty-five pounds and are poured into a resin bonded shell mold in the vertical position using either a top or bottom pouring mold. Immediately prior to the pouring of the mold, about three quarters of one ounce of fused borax in granular form is added to the mold. When this mold is poured and is permitted to cool and then fractured, a complete absence of the cope defect will be found.

The above procedure is satisfactory where a fine finish is not required on the rough casting. However, the borax or other magnesium oxide flux has an erosive effect upon the sand of the mold and results in a casting having a somewhat rougher finish than would be obtained in the absence of the flux. This disadvantage may be avoided by treating the metal in the ladle with the magnesia flux. A typical ladle treatment requires theuse of about eight pounds of magnesia flux per ton of metal. The flux should be permitted to remain in contact with the metal long enough and agitated sufliciently to insure the proper degree of elimination of magnesium oxide. This treatment may be supplemented by a subsequent flux treatment in the mold, if required.

While not desiring to be bound by the accuracy of their theory, it is the inventors belief that the presence of the cope defects is traceable to the presence of magnesium oxide in the casting and that the elimination of these de-- fects is a consequence of the removal of the magnesium oxide by the magnesium fluxes.

We claim as our invention:

1. In the production of nodular cast iron, the steps comprising adding to a quantity of molten iron sufficient magnesium to yield a nodular as cast product, treating the magnesium bearing molten metal with a flux having the property of wetting magnesium oxide, and casting the molten metal into a mold, said molten iron being protected from oxidizing gas after treatment with the flux.

2. In the production of nodular cast iron the steps 3 4 Comprising adding to a quantity of molten iron sulficient molten iron in a mold and protecting the molten flux treatma'gn'esiu'm to yield a nodular as cast product, treating ed metal from oxidizing gas prior to solidification.

the magnesium bearing molten metal with a flux chosen from the group consisting of borax, boric acid, fused References Cited in the file of this patent horas, alkali fiuoborates and mixtures containing alkali 5 UNITED STATES ATENTS metal chloride and magnesium chloride, and casting the molten metal into a mold, said molten metal being pro- 2'265740 Morgan 1941 2,378,699 Gunn June 9, 1945 tected from oxidizing gas after treatment with the flux.

2,518,738 Woods et a1. Aug. 15, 1950 3. In the production of nodular cast ll'OIl, the steps of treating molten ma nesium containing iron in a ladle with 10 2'527498 Jorda-n 1950 5 2,574,581 McKinney et al Nov. 13, 1951 a tlux capable of wetting magnesium oxide, casting the 

1. IN THE PRODUCTION OF NODULAR CAST IRON, THE STEPS COMPRISING ADDING TO A QUANTITY OF MOLTEN IRON SUFFICIENT MAGNESIUM TO YIELD A NODULAR AS CAST PRODUCT, TREATING THE MAGNESIUM BEARING MOLTEN METAL WITH A FLUX HAVING THE PROPERTY OF WETTING MAGNESIUM OXIDE, AND CASTING THE MOLTEN METAL INTO A MOLD, SAID MOLTEN IRON BEING PROTECTED FROM OXIDIZING GAS AFTER TREATMENT WITH THE FLUX. 